Alchemists tried in vain to transmute lead into gold. What if you could turn waste products into energy? That’s what [chemicum] did in a recent video–he and some friends built microbial fuel cells that convert excrement into electricity (you can see the video, below).
The video doesn’t give you all the details of the build, but it seems easy enough. You need some stainless steel mesh, some activated charcoal, some epoxy, plastic containers, and some assorted metal plates and hardware. Of course, you also need excrement and–if the video is any indication–some clothespins to clamp your nose shut as you work.
The resulting cell produced a bit over 800mV at 13mA. You can, of course, add more cells to get more voltage or current. Unlike batteries, fuel cells create power as long as they have a source of fuel. The cells in the video don’t appear to have any way to renew their fuel supply. If you want more detail about bacteria-based fuel cells, there’s always Wikipedia.